This invention relates to the coating of articles, and more specifically to an apparatus for and a method of applying a chromized coating to one or more of the surfaces of an article as a means of providing protection thereto particularly with respect to corrosion.
It has long been known in the prior art that from the standpoint of being able to resist various forms of corrosion the most important alloying element insofar as steel is concerned is probably chromium. Furthermore, it has also long been known in the prior art that one means of providing the levels of chromium which are necessary in order to render steel resistant to such corrosion and/or oxidation is through the use of that process to which heretodate the term "chromizing" has commonly been applied in the prior art.
As used in the prior art, the term chromizing has come to mean a high temperature diffusion process in which the treated surface or surfaces of a steel object are alloyed with chromium. Briefly stated, in accord with this process the steel object is submerged in a retort in powder that contains chromium, the retort is sealed and is then heated along with its contents in a furnace for several hours at an elevated temperature. This heating causes the chromium in the powder to gasify, to deposit on the steel object, and to diffuse into the base metal of the steel object to a depth and in a concentration that is dependent upon numerous metallurgical and process variables. From the foregoing process there is produced a steel object which embodies an iron-chromium alloy coating that is metallurgically bonded as an integral part of the base metal of the steel object. Being that chromizing is a diffusion process, the structural modifications which result from the use of this process occur within the surface of the base metal and not on the surface itself. The significance of this is that since the diffused chromium has physically and metallurgically penetrated the base metal surface of the steel object, substituting for some of the iron atoms of the base metal in doing so, the diffused chromium is an integral part of the base metal of the steel object, and therefore, is not subject to spalling or peeling that characterize mechanical bonded coatings.
There are known to exist a number of different types of applications where it would be desirable to utilize components that have been chromized, i.e., that have had a chromium coating applied thereto. In this connection, by way of exemplification and not limitation, reference is had here in particular to various boiler related applications which are known to give rise to situations wherein there exists a need to combat the many forms of general and/or localized gas side corrosion and solid particle steam cycle erosion caused by steam side oxide scale exfoliation.
Chemical recovery boilers are an example of one such boiler related application. A chemical recovery boiler essentially is an apparatus for recovering and processing certain chemicals which are used in a pulp mill. Of particular interest insofar as chemical recovery boilers are concerned is the so-called "black liquor", i.e., the spent cooling liquid that is generated during pulp making. It is this black liquor which is burned as fuel in the chemical recovery boiler. For many years the black liquor was burned in the chemical recovery boiler as a means of recovering chemicals before use was also made thereof to generate steam. The value of the chemicals that are recovered through the use of the chemical recovery boiler is normally on the order of three times greater than that of the steam generated by the chemical recovery boiler. Nonetheless, the steam generated by the chemical recovery boiler is a substantial amount i.e., perhaps half, of the steam that a pulp mill requires. However, unfortunately the black liquor which is burned as fuel in the chemical recovery boiler produces an environment which has proven to be highly corrosive to the carbon steel waterwalls of the chemical recovery boiler. As a result, for purposes of resisting this corrosion the boiler tubes which are employed in the waterwall panels of the chemical recovery units heretofore have either had to be replaced frequently and/or metallized, both necessitating a shutting down of the chemical recovery boiler in order to provide access thereto. Another option can be special composite tubes of carbon steel with an outside clad of Type 304 stainless steel but such tubes are relatively costly. An alternative thereto would be to employ boiler tubes that have been chromized, i.e., boiler tubes that have had a chromium coating applied thereto.
Another example of a boiler related application for which chromized components, i.e., components to which a chromium coating has been applied, appear to be particularly suited for use is that of resource recovery boilers. Because of the ever diminishing number of landfills that remain available for receiving municipal waste, more and more communities are resorting to burning their municipal waste. One means of accomplishing this burning of municipal waste that is being employed by such communities is through the use of a boiler of the type which is most frequently referred to by those in the industry as a resource recovery boiler. Unfortunately, however, it has been found that by virtue of its composition, i.e., because of the nature of the various types of materials that are to be found contained in municipal waste, the burning of such municipal waste in a resource recovery boiler frequently gives rise to a situation wherein at least some of the boiler tubes thereof are subjected to corrosion. Inasmuch as the burning of municipal waste in a boiler on the scale that is now being attempted represents a relatively recent development in terms of the types of applications in which boilers have historically been employed, there remains much to be learned with respect to the nature of the corrosion to which the boiler tubing of a resource recovery boiler will be subjected as a result of the burning of the municipal waste in the resource recovery boiler as well as with respect to the extent to which such boiler tubing of the resource recovery boiler will be subjected to this corrosion. Notwithstanding this though, there exists sufficient information which would appear to indicate that it would be possible to successfully resist such corrosion by utilizing chromized boiler tubing in the resource recovery boiler, i.e., boiler tubing that has had a chromium coating applied thereto.
Yet another example of a boiler related application for which chromized components, i.e., components to which a chromium coating has been applied, appear to be particularly suited for use is that of the gasifier which is designed to be employed in a coal gasification process. There existed considerable interest in the U.S., particularly during the 1970's, in developing a commercial version of a coal gasification process for producing synthetic gas from coal. In support of such a developmental effort funding was made available during this period not only by the U.S. government but also by a number of large U.S. companies. The level of interest in the U.S. in pursuing coal gasification on a commercial scale subsided significantly with the subsequent drop in oil prices. On the other hand, although the level of interest in pursuing coal gasification on a commercial scale remains relatively low at the present time in the U.S., considerable interest has relatively recently been evidenced in coal gasification overseas. As a consequence in particular of this latter interest attention is now being focused on ways to combat the corrosion to which it has been known the internal components of the gasifier are subjected in the course of the operation of the gasifier. As in the case of the resource recovery boiler discussed previously herein, there remains much to be learned with respect to the nature of the corrosion to which the internal components of the gasifier are subjected in the course of the operation of the gasifier as well as with respect to the extent to which the internal components of the gasifier will be subjected to this corrosion. Notwithstanding this though, sufficient information does exist from which a conclusion can be drawn that it appears that it would be possible to successfully resist such corrosion by chromizing internal components of the gasifier, i.e., by applying a chromium coating thereto.
Coal-fired utility boilers represent still another example of a boiler related application for which chromized components, i.e., components to which a chromium coating has been applied, appear to be particularly suited for use. To this end, it is known that coal-fired utility boilers, especially of the supercritical pressure type, have suffered general and/or localized attack of the waterwalls thereof. Moreover, metallizing has not proven to be effective in combating this very aggressive sulfidizing environment. On the other hand, there are indications that components which have had a chromium coating applied thereto are resistant, even when located in areas in which severe metal loss has previously been encountered, to such sulfidation attack and metal loss.
Continuing with the discussion of boilers, the superheater and the reheater sections thereof have been known to suffer from high temperature ash corrosion and oxidation. This high temperature ash corrosion and oxidation in the superheater and reheater sections of such boilers can be addressed in some instances by upgrading the material from ferritic steel to austenitic stainless steel. However, due to the differing coefficients of thermal expansion of ferritic and austenitic stainless steel, there are other instances wherein it is desirable, or even necessary, to continue to utilize a ferritic steel as the material from which some or all of the components in the superheater section and/or the reheater section of the boiler are fabricated. In cases such as these, it appears that the use of chromizing, i.e., applying a chromium coating to such ferritic alloy components, will be of benefit in providing resistance to ash corrosion and will also significantly raise the oxidation limit of the base metal of these components, i.e., of the ferritic alloy material from which these components are fabricated. With further reference to the matter of differing coefficients of thermal expansion, the fact that materials expand at different rates is particularly a concern in retrofit applications wherein because the original design was based on the use of ferritic material it may not be feasible to now replace the ferritic material with austenitic material or vice versa, i.e., where the original design was based on the use of austenitic material to now attempt to replace the austenitic material with ferritic material. Thus, there exists the need to employ components fabricated from ferritic alloy materials. However, in doing so there also exists a need to do something to raise the oxidation limit of the ferritic alloy material. Such a result can be had by employing components fabricated from ferritic alloy material that have been chromized on the outside and the inside, i.e., have had a chromium coating applied thereto on the outside and the inside thereof. By doing so, a component would be provided that would be expected to have a significantly longer life than a component fabricated from ferritic alloy material which had not been chromized.
With further reference to the matter of oxidation, the steam side oxidation product that forms on the tubes in the superheater section and the reheater section of the boiler which are touched by gas is known to have a potential for producing significantly increasing metal temperatures. Moreover, there have been boiler tube failures attributed to the formation of heavy internal oxides on the tube surfaces. By way of contrast, the oxide scale that forms on surfaces to which a chromium coating has been applied have been found to be extremely thin. Consequently, because the oxide scale is extremely thin, the metal temperature of boiler tubes fabricated from ferritic alloy material that have been chromized, i.e., have had a chromium coating applied thereto, will not increase significantly even when subjected to hours of operation.
Reference will next be had herein to the matter of a form of metal wastage often referred to as corrosion fatigue, circumferential grooving, elephant hiding, etc. With regard thereto boiler tubes that have the fire sides thereof, i.e., the sides thereof which are presented to the fire in the boiler, exposed to a high front to back temperature differential, e.g., usually those that are exposed to radiant heat, are sometimes found to have suffered circumferential cracking. While the cause of such circumferential cracking is not completely understood it is believed that it is induced by thermal strains. The mechanism by which the penetration of the base metal, i.e., the cracking thereof, occurs is as a consequence of the repeated cracking of the semi-protective oxidation product formed on the boiler tube surface. The waterwall tubes of coal-fired utility boilers, especially of the supercritical pressure type, that experience sulfidation wastage attack are known to also experience corrosion fatigue cracking particularly in those areas thereof which are outside of the regions wherein severe metal loss is experienced. Further, it is known that radiant wall tubes in the reheater section of coal-fired boilers have also experienced this type of cracking, i.e., thermally induced corrosion fatigue cracking. In this case also, chromizing such boiler tubes, i.e., applying a chromium coating to the surface thereof, has shown itself to be quite resistant to the aforereferenced type of corrosion fatigue cracking, i.e., the thermally induced corrosion fatigue cracking which has been discussed above.
To thus summarize, based on the preceding discussion it should now be readily apparent, by way of exemplification and not limitation, that numbered among the various types of applications wherein there is a need to provide materials with protection against the oxidation and sulfidation which is known to occur on heat transfer surfaces and wherein applying a chromium coating to, i.e., chromizing, the surfaces of such materials has been found to be effective to provide such protection are those in the electric utility, paper, petrochemical, coal gasification and chemical process industries.
Continuing, as noted herein previously, it has long been known in the prior art to apply a chromium coating to the surface of a material. To date though the application of such coatings of chromium to the surface of a material have been disadvantageously characterized in two respects primarily. One of these is the fact that particularly insofar as the application of a chromium coating to the external surface of a component is concerned, it has heretofore only been possible to coat with chromium the external surfaces of relatively small size components, e.g., turbine blade size components. The reason for this stems from the fact that there is a need in applying a chromium coating to the surface of a material that this be done at a specified temperature. However, the larger the component the more difficult it is to maintain at this specified temperature the entire surface or surfaces of the component that it is desired to coat with chromium for the length of time required to effect the application of the coating of chromium thereto.
Notwithstanding whether the coating of chromium is applied to the external or the internal surface of the material the other disadvantage from which prior art methods of applying a chromium coating to the surface of a material have suffered is that the chromium coating which results therefrom is characterized in that it contains a large preponderance of grain boundary carbides. Reference is had here in this regard to the methods which form the subject matter of U.S. Pat. No. 4,208,453 which issued on June 17, 1980 and U.S. Pat. No. 4,290,391 which issued on Sep. 22, 1981. The reason why the presence in a chromium coating of a large preponderance of grain boundary carbides is considered to disadvantageously characterize the latter is that it has been found that such grain boundary carbides can cause the chromium coating to be susceptible to intergranular attack in some service environments. Efforts have been undertaken to overcome this disadvantage occasioned by the presence of a large preponderance of grain boundary carbides in a chromium coating through the minimization of the presence of grain boundary carbides therein. One way in which it has been sought to accomplish such minimization of the presence of grain boundary carbides in the chromium coating is to codiffuse vanadium with the chromium. The concept of codiffusing vanadium with the chromium forms the subject matter of U.S. patent application Ser. No. 68,922, which was filed by the Electric Power Research Institute (EPRI) on July 1, 1987 and which lists as one of the co-inventors thereof, E. Clyde Lewis, one of the inventors of the subject matter to which the present patent application is directed. Another way has been by using carbon stabilized low alloy base material as the material to which the coating of chromium is applied.
A need has, thus, been evidenced in the prior art for a new and improved apparatus and method for applying a coating of chromium to one or more of the surfaces of an article, be the surfaces the external or the internal surfaces thereof, particularly in those situations in which the article is intended to be employed in an application wherein the article will be subjected to corrosion. In addition, a need has been evidenced in the prior art for such an apparatus and method for applying a coating of chromium to the surface of an article wherein the chromium coating applied to the surface of an article in accord therewith is thicker than that which it has heretofore been possible to provide through the use of prior art chromium coating methods. Moreover, there has been evidenced in the prior art a need for such an apparatus and method for applying a coating of chromium to the surface of an article wherein the chromium coating applied to the surface of an article in accord therewith contains a higher concentration of chromium than that which heretofore it has been possible to achieve through the use of prior art chromium coating methods. Furthermore, there has been evidenced in the prior art a need for such an apparatus and method for applying a coating of chromium to the surface of an article wherein the article to the surface of which in accord therewith the chromium coating is applied is larger in size than the size of the articles to which it has heretofore been possible to apply a chromium coating to the surface thereof through the use of prior art chromium coating methods.
It is, therefore, an object of the present invention to provide a new and improved apparatus for applying a coating of chromium to one or more of the surfaces of an article as a means of protecting the latter.
It is another object of the present invention to provide a new and improved method of applying a coating of chromium to one or more of the surfaces of an article as a means of protecting the latter.
It is still another object of the present invention to provide such an apparatus and method for applying a coating of chromium to one or more of the surfaces of an article wherein the surfaces to which the coating of chromium is applied may be the external and/or the internal surfaces of the article.
A further object of the present invention is to provide such an apparatus and method for applying a coating of chromium to the surface of an article wherein the chromium coating that is applied in accord therewith is thicker than that which it has heretofore been possible to provide through the use of prior art chromium methods.
A still further object of the present invention is to provide such an apparatus and method for applying a coating of chromium to the surface of an article wherein the chromium coating that is applied in accord therewith contains a higher concentration of chromium than that which heretofore it has been possible to achieve through the use of prior art chromium coating methods.
Yet another object of the present invention is to provide such an apparatus and method for applying a coating of chromium to the surface of an article wherein the article to the surface of which in accord therewith the chromium coating is applied is larger in size than the size of the articles to which it has heretofore been possible to apply a chromium coating to the surface thereof through the use of prior art chromium coating methods.
Yet still another object of the present invention is to provide such an apparatus and method for applying a coating of chromium to the surface of an article which are relatively inexpensive to provide, which are relatively easy to employ and which are characterized in that the chromium coating provided thereby is better capable of resisting corrosion than the chromium coatings which heretofore have been available for use for the same purpose.